Cortical interneurons as a population are recognized to be remarkably diverse in terms of their morphology, connectivity and physiological properties. In recent years numerous investigators have focused on understanding how this diversity is generated. In the previous funding cycle of this grant we were able to demonstrate that the place and time of origin of different cortical interneuron populations predicts their mature properties (Butt et al., 2005). In particular we found that the medial and caudal ganglionic eminences (MGE and CGE, respectively), while together accounting for the vast majority of cortical interneurons, produce entirely non-overlapping cohorts. In an effort to understand the developmental genetic mechanisms by which different cortical interneuron populations arise from these structures, we undertook a conditional loss of function analysis of the homeobox-containing gene Nkx2-1, which at present is the only gene that precisely distinguishes between the MGE and CGE (Butt et al., 2008). This study revealed that Nkx2-1 acts as a molecular toggle switch that promotes the generation of MGE-derived cortical interneuron populations and represses the CGE-derived interneuron cell identities. In this grant we will explore the genes that are both positively and negatively regulated by Nkx2-1 gene function. First we will undertake a structure function analysis of Nkx2-1 to examine its ability to act as a transcriptional activator and repressor. We will follow this by exploring the contribution of genes that are activated by Nkx2-1 and evaluate their contributions to the production of cortical interneurons with specific subtype character. Finally we will use a genetic approach to explore the diversity and timing of CGE-derived interneuron generation and how CoupTF1 and CoupTF2, CGE-expressed genes with complementary expression to Nkx2-1, contribute to the generation of the interneuron subtypes derived from this structure. Together, this study will provide insights into the molecular basis by which cortical interneuron subtypes are generated. Increasingly it is being recognized that cortical interneurons through their maintenance of the excitatory/inhibitory balance in the CNS are central to the normal function of the nervous system. In addition, cortical interneurons have been implicated in neurological disorders including epilepsy, bipolar disorders and autism. Our proposal by exploring the genetic mechanisms by which these cell types are generated has the potential to ultimately provide tools for targeting and manipulating this critical population.